Due to the high surface power density of the excitation-laser beam spot on the luminous material provided for the conversion and the Stokes-shift effect occurring during the wavelength conversion, these fluorescent devices are usually constructed in the form of a freely rotating “fluorescent wheel”. In this arrangement, the luminous material is applied to one side of a rotating disk. As a result, the momentarily irradiated fluorescent part area turns away under the excitation-laser beam spot before permanent local damage or even destruction of the luminous material occurs. Because of the rotation of the fluorescent wheel, the irradiated area describes an annular track after each complete rotation. The luminous material is therefore usually applied in the form of an annular track on one side of the fluorescent wheel. The heating of the luminous material caused by the excitation-laser radiation in the course of the operating time is transferred to the environment via the rear of the fluorescent wheel, among other things (radiation and convection cooling). Such fluorescent devices and reflector lamp arrangements equipped therewith are suitable for many applications which require light sources having a high intensity of illumination such as video projection, medical and industrial endoscopy by means of optical waveguides, effect lighting, car headlamps etc., and which have hitherto been the domain of reflector lamps based on halogen incandescent lamps or discharge lamps.
Document WO 2009/112961 A1 describes a laser light source which has at least one laser light emitting element, at least one light source output element (which is configured for directing the laser light onto a predetermined location), at least one conversion element and one reflector arrangement. The at least one conversion element includes a luminous material which converts the laser light at least partially into wavelength-converted or converted light. The reflector arrangement combines the converted light and the laser light to form a mixed output light.
Document US 2009284148 A1 discloses a light source device for a projector including an excitation-light source and a rotating fluorescent wheel. The fluorescent wheel has a transparent circular disk. On one side of the disk, a segmented annular track is arranged. The track is divided into three equally large segments, namely into one luminous material emitting in the red wavelength range (R), one luminous material emitting in the green wavelength range (G) and one diffusely transparent area (135). The excitation light impinges on the rear of the rotating fluorescent wheel, passes through the transparent disk and excites the luminous material of the annular track on the other side of this disk. The fluorescent wheel is thus used in the so-called transmissive mode. In the axis of the incident excitation light, an optical waveguide which forwards the converted light is arranged opposite the annular track.
In previous fluorescent wheels, inadequate cooling for preventing or at least delaying for a sufficiently long period a degradation of the luminous material excited by means of the excitation-laser radiation is disadvantageous, especially with increasing excitation-laser powers. This problem is intensified in the case of fluorescent wheels coated on both sides, not only because the heat dissipation via the rear of the fluorescent wheel is distinctly reduced but because, instead, excitation-laser radiation also impinges on the rear coated with luminous material, including the associated additional heating.